12 y “H" 2 b MM+dM dM M2 s小dA=r,btx 2 M2-m1-bx=0 s dM M FM o, dA=J My da= dx lb dx FS F2=,M (M+dM da 1bb y y z 2 h 2 h F a a dA 1 y  A  FN1  FN 2 x dx 1 1 2 2 1 2 a a y  y M M +dM y  a a 1 2 dx b 0 * 1 * FN2 − FN − y bdx =  = * 1 * 1 A FN  dA  = * 1 A z dA I My  = * 1 A z y dA I M  = * 2 * 2 A FN  dA ( )  + = * 1 A z dA I M dM y y dA bdx I dM y A z =  * 1  z S dx dM I b S z z y *  = I b F S z s z *  =